This invention relates to electronic cancelling of acoustic traveling waves, and more particularly to the cancellation of relatively low to medium frequency range acoustic waves in a thin rod.
One field of use is the isolation of a strong acoustic source, such as a high energy laser device, from an acoustically fragile structure.
Any object that vibrates and disturbs its surrounding ambient medium may become an acoustic source by radiating acoustic waves which vary in wavelength. Very often, the vibration is unwanted and is a source of acoustic noise. The prior art has been concerned with such noise as may be radiated for example from reverberating structures, vibrating machinery, large transformers and various other large types of apparatus in various ambient mediums.
The most direct means for reducing the sound intensity from a typical acoustic source is to surround the source with an acoustic baffle which cuts off its direct acoustic propagation path. Various absorbing materials exist which have the ability to dissipate sound energy to heat energy. Such absorbers work well for the high frequency range, however, they are extremely bulky and limited in application for the low frequency range.
There is also a common problem in systems such as gimbals that have a torquer drive motor separated from the rate and position sensors by the mechanical structure of the gimbal. If the gimbal is flexible and has one or more resonant modes, then resonant peaks will be seen in the torquer response. If the frequency of the resonance is low enough, and the mechanical Q of the structure is high enough, this resonance can be a source of torquer control loop oscillations and sometimes even result in a loss of loop control. Even if the resonance problem is not severe enough to produce oscillation, it can still have a significant effect on the torquer response. As a result, the speed of response of the torquer control loop is usually limited by the lowest resonant frequency of the gimbal. The gimbal mechanical resonance problems are usually attacked by mechanical solutions, such as stiffening the structure, decreasing the inertia, or applying resonant or nonresonant dampers.
The various solutions mentioned above are passive techniques. There are also many known active techniques, using some form of electronic damping. A common type of noise cancellation arrangement employs a microphone, amplifier and loudspeaker to measure the noise and to produce equal amplitude and opposite phase acoustic signals to cancel out the sound.
In the case of the gimbal mechanical resonance problems, one electronic damping technique has energy inserted directly into the structure by a piezoelectric drive transducer, as reported in a paper by R. L. Forward, "Electronic Damping of Vibrations on Optical Systems", Applied Optics 18, 690-697 (1979). In another technique, the gimbal motor is used to supply the final power stage of the feedback damping loop. This electronic damping concept for control of gimbal torquer resonances involves placing a sensor (usually a piezoelectric strain transducer) on the mechanical structure to sense the impending excitation of the resonant mode by the forces from the torquer drive motor. These electrical signals are then amplified, phase shifted, and fed back into the torquer drive electronics at an appropriate summing point. With a proper choice of gain and phase, the modified torquer system operates as before, except that the resonant response has been reduced in amplitude and broadened in frequency just as if a mechanical damper had been placed on the gimbal structure.
R. L. Wanke in U.S. Pat. No. 3,936,606 describes an "acoustic Abatement Method and Apparatus". He points out that an antiwave which is 180.degree. out of phase with respect to an acoustic wave, although it will cancel the intermediate portion of a pure sine wave, it will not cancel the first half cycle of an acoustic wave, nor the last half cycle of a locally generated antiwave. When the acoustic wave has a nonsymmetrical pressure variation, a 180.degree. phase shift does not cancel the acoustic wave but in fact adds to the total objectionable sound energy. He alleges that complete cancellation by wave interference requires the use of an antiwave which is in phase and of mirror symmetry with respect to the acoustic wave to be cancelled. His patent covers a system for generating and introducing a mirror symmetry antiwave into the gas flow of a gas turbine.
Swinbanks Pat. No. 4,044,203 discloses a system for active control of soundwaves wherein sound sources spaced along a duct generate two waves traveling in opposite directions. Those waves traveling in the same direction as the unwanted wave sum to give a result which interferes directly with the unwanted wave while those traveling in the opposite direction sum to give a negligible result.
Coxan et al U.S. Pat. No. 3,602,331 discloses an apparatus for attenuating a sound wave propagating in a given direction along a duct.
Other references which describe several techniques for active cancelling of sound in ducts are: (1) J. H. B. Poole and H. G. Leventhal, "An Experimental Study of Swinbank's Method of Active Attenuation of Sound in Ducts", Journal of Sound and Vibration, 49 (2), 1976, pp. 257-266; (2) H. S. Leventhal, "Development in Active Attenuators", 1976 Noise Control Conference, Warsaw, Oct. 13-15, 1976; and (3) J. H. B. Poole and H. G. Leventhal, "Active Attenuation of Noise in Ducts", Journal of Sound and Vibration, 57 (2), 1978, pp. 308-309. These techniques use delay lines and phase shifters to phase shift the signal recorded by a microphone. High fidelity loudspeakers inject these phase shifted signals into the duct to actively cancel the incident sound.
Unfortunately, the reported frequency range of operation is relatively limited for these active cancelling techniques. Attenuation of 20 dB is reported over approximately an octave, e.g., 100 to 200 Hertz or 80 to 160 Hertz with about 35 dB attenuation at a selected frequency.
Patents which disclose other sound control systems and techniques of interest include Andre et al, U.S. Pat. No. 4,255,083, Angelini et al U.S. Pat. No. 4,177,874, Davidson U.S. Pat. No. 4,025,724, Bschorr U.S. Pat. No. 3,602,331, Behrend U.S. Pat. No. 4,096,454, and McCormack U.S. Pat. No. 3,757,235.
In general, techniques of the prior art provide moderate cancellation of steady state vibrations at selected frequencies, or over small frequency bands.